Thanks for the link ejrolon.
Turns out in my case it had to do with the complex profile I was using for the wall. It was a cavity wall: CMU, rigid insulation, air space and brick. I didn't use a fill for the air space. I just placed the brick fill 2" away from the insulation fill.
Apparently Archicad doesn't like profiles in which the fills don't touch (at least when using Priority Based Connections). When I put a empty fill in for the air space, rather than just a gap between fills, the problem went away.
IMac OS X 10.13.5
4 core Intel 3.4 Ghz, 16 GB Ram,
AMD Radeon HD 6970M
Archicad 15, 16, 17, 18, 19, 21, 23